1. Field of the Invention
The present invention relates to a switching element, a method for manufacturing a switching element, and a display device including a switching element. In particular, the present invention relates to a display device including a mechanical switch.
2. Description of the Related Art
In recent years, as a display device using a liquid crystal panel or the like, an active-driving display device using thin film transistors (TFTs) is employed in many cases for higher definition. In the case of an active-driving display device, the potential of each pixel electrode can be independently controlled and thus there is no crosstalk such as a leak of an electrical field to an adjacent pixel, as there is in the case of a passive-driving display device; therefore, a panel with less unevenness in display and a higher contrast ratio can be manufactured.
However, active driving has a problem in that because switching on/off of a potential of a pixel electrode is performed electrically using TFTs, an off current (leak current) flows even in an off state and thus it is difficult to maintain a completely off state. When there is an off current, it is difficult to hold a potential of a pixel electrode. Therefore, it is necessary to provide an additional storage capacitor. Further, the higher an off current is, the larger the storage capacitor needs to be. Thus, problems such as lower driving frequency and a flicker in a screen are caused. Further, because it is necessary to supply an excess charge to the storage capacitor, power consumption increases.
In order to solve the above problems, a display device including a mechanical switch as a switching element has been proposed (for example, Cited Document 1: Japanese Published Patent Application No. H9-92909 and Cited Document 2: Japanese Published Patent Application No. 2000-35591). A mechanical switch is operated instead of a switching element such as a transistor, by displacing a fixed flat spring by electrostatic force so that a conductive film (upper electrode) provided at a top portion of the flat spring and a conductive film (lower electrode) such as a pixel electrode are or are not in contact with each other. Further, a display device in which an upper electrode and a pixel electrode come or cease to be in contact with each other depending on contraction of a piezoelectric element has been proposed (for example, Cited Document 3: Japanese Published Patent Application No. H11-174994).
However, in the case where a switching element is provided so as to have a plate structure in which one side is fixed (Cited Document 1), stress is locally concentrated at the time of switching operation of the switching element (a contact or noncontact operation between an upper electrode and a lower electrode); therefore, damage of the switching element is a problem. Similarly, in the case where a flexible thin film and a conductive film are bent with the use of a supporting board to perform a contact or noncontact operation (Cited Document 2), stress is also caused in a specific portion of the flexible thin film; therefore, resistance of an element is a problem. Further, it is necessary to form a somewhat large gap; therefore, it is difficult to miniaturize the switching element.
As for the structure of a piezoelectric switch disclosed in Cited Document 3, even when a switching element is off (an upper electrode and a pixel electrode are not in contact with each other), a potential difference is generated in a piezoelectric element as a signal line which functions as a lower electrode changes, so the switching element cannot be completely turned off, and therefore, malfunction might occur. In particular, in a case where a gap between the upper electrode and the pixel electrode is small, there is a significant possibility that malfunction might occur.
Meanwhile, in the case where the gap is widened to prevent malfunctions, in order that the switching element be turned on, it is necessary for a potential difference between the upper electrode and the lower electrode to be larger, and thus, power consumption of the switching element increases. In this case, a potential difference between a wiring connected to the upper electrode and a wiring connected to the lower electrode is also larger; therefore, there is a possibility that a leak current might occur in a portion where the wirings intersect with each other. Consequently, a display defect might be caused due to reduction in voltage applied to the piezoelectric element as well as increase in power consumption.